The present invention relates generally to noncontact sensors and to an apparatus and method for selectively energizing and de-energizing inductors in a system which enables the use of low cost digital circuitry to provide a transient free output waveform. The invention may be used in a switched complementary sensor coil application in which the coil inductances vary in response to position of a moving member.
Noncontacting sensors typically consist of two or more tunable inductors each. Although circuit configurations vary, usually the tunable inductive element is connected alternatively to a capacitor to form a resonant tank circuit. Typically, two complementary inductor coils are used in a switched arrangement.
In the conventional switched arrangement a first coil is connected to the tank circuit and energized while a frequency measurement is taken. Thereafter, the first coil is disconnected and a second coil is simultaneously connected to the tank circuit where a second frequency measurement is taken. Switching from one coil to the other in this fashion using digital switching techniques results in transients, which are a source of error. This includes an arrangement whereby the coils are multiplexed at a task repetition rate.
The present invention provides a substantial transient reduction during coil switching using a staggered coil activation technique. The technique allows the coils to be multiplexed after no longer than one period, affording a system with much better frequency response than is possible using conventional switching techniques. The invention is well adapted to implementations using low cost, high speed digital components. The staggered technique offers a system which is quite tolerant to variations in signal propagation delays common in digital circuit components. In other words, unlike conventional switching techniques, the invention is able to properly function even if the signal for energizing the first coil and the signal for de-energizing the second coil propagate through different circuit paths and therefore have different arrival times.
The present inventive method may be used to indicate the inductance of a system of inductors which includes a first inductor and a second inductor, for example. The inductance may be correlated to a measured parameter such as position of a tuning slug or tuning plate. Hence, the invention is applicable to a wide range of different parameter sensors.
The method comprises coupling the first inductor to a tank circuit and thereafter supplying energy to the tank circuit to cause oscillation at a first resonant frequency which is determined at least in part by the first inductor. A waveform indicative of the first resonant frequency is produced in this fashion. Next, the second inductor is coupled to the tank circuit without decoupling the first inductor from the tank circuit, resulting in a coil overlap period. Thereafter, the first inductor is decoupled from the tank circuit, without decoupling the second inductor. Energy is then supplied to the tank circuit to cause oscillation at a second resonant frequency which is determined by the second inductor followed again by a coil overlap period. A waveform indicative of the second resonant frequency is thus produced. The waveforms indicative of the first and second resonant frequencies produced in this fashion are indicative of the coil inductance.
The invention affords numerous advantages over conventional switching techniques. It ensures that at least one coil is always connected in the tank circuit, thereby avoiding errors resulting from discontinuities. Only one coil changes status at a given time, never both at once allowing for smooth transitioning. Further, the coil being read is not subject to change during its entire measurement cycle, allowing a transient free measurement. Also, by connecting both coils together between individual coil reading cycles all transients are absorbed.
For a more complete understanding of the invention and its further objects and advantages, reference may be had to the following specification and to the accompanying drawings.